This application claims the benefit of Japanese Patent Application Number 2005-101479 filed on Mar. 31, 2005, the entirety of which is incorporated by reference.
1. Field of the Invention
The present invention relates to a method for correcting thermal displacement that varies depending on a position of a tool or a workpiece in a machine tool.
2. Description of the Related Art
In a machining process of a machine tool, the machine tool and a workpiece are subject to thermal deformation due to changes of the environmental temperature of a room, heat emitted from the machine tool, and cutting heat. This thermal displacement leads to machining dimension error. As a method to reduce the dimension error, from a mechanical or structural point of view, methods to maintain a constant temperature of the machine and the workpiece have been proposed, for example, by installing the machine in a temperature-controlled room, and controlling the temperature of the cutting fluid. However, these methods require higher running costs and further have a problem in that it is difficult to set a standard temperature of the cutting fluid.
On the other hand, from the viewpoint of electrical control, a method to estimate and correct thermal displacement from temperature information or NC unit information has been proposed. The applicant of the present invention disclosed an example of this method in Japanese Published Unexamined Patent Application No. 2001-341049. This particular method includes the steps of detecting the coordinate data of machining point, that is, the coordinate data of the cutting edge position in machining (hereinafter, it is simply referred to as “cutting edge coordinate data”), and estimating thermal displacement in accordance with the coordinate data of the machining point and temperature information.
Japanese Published Examined Patent Application No. 06-61674 discloses another method that estimates the thermal displacement in accordance with the difference between a main spindle position and a standard position, as well as temperature information. Further, a document Katagijutsu (that means “die/mold technology”), Volume 18, Issue 8, Pages 44-45 discloses another method for estimating thermal displacement generated on a table of a double-column machining center in accordance with temperature information and positional information.
The above-described methods for estimating thermal displacement in accordance with temperature information or positional information from an NC unit work effectively only when a workpiece is cut on a lathe in which the fixed position of a workpiece is defined as the center of the main spindle, or only when a small workpiece is disposed on a table of a machining center and so on. However, when a relatively large workpiece such as a mold is cut, there is a problem that the thermal displacement of the workpiece varies depending on its fixed position on a table.
FIG. 9A is a schematic side view of a conventional double-column machining center. Columns 6, 6 are disposed on opposite sides of a bed 1 (in the direction orthogonal to a drawing sheet). Between the columns 6, 6, a cross rail (not shown) is bridged transversely, and a saddle 7 is mounted on the cross rail movable in the Y-axis direction orthogonal to the drawing sheet. A main spindle 8 is attached on the saddle 7 movable in the Z-axis direction that is vertical. At the distal end of the main spindle 8, a tool 9 is rotatably connected. Above the bed 1, a table 4 is disposed movable in the X-axis direction parallel to the drawing sheet, and a workpiece 5 is fixed on the table 4.
In FIGS. 9B and 9C, each cutting edge coordinate data Xa is shown at the same position. On the other hand, each coordinate data of the fixed position of the workpiece Xw is shown in different positions, for example, Xwa in FIG. 9B and Xwb in FIG. 9C. FIG. 11 shows thermal displacement change in the X-axis direction in accordance with the fixed positions in FIGS. 9A-9D at the room temperature shown in FIG. 10. FIG. 11 shows that change of the thermal displacement varies depending on the coordinate data of the fixed position even if the cutting edge coordinate data is unchanged. For this reason, in order to obtain the thermal displacement of the workpiece based on the cutting edge coordinate data only, it is necessary to change correction parameters in accordance with the coordinate data of the fixed position.
Moreover, in machining a relatively large workpiece such as a mold, when the workpiece is disposed at a place whose environmental temperature is different, there might be another problem in that the dimension and configuration of the workpiece might be different from the desired ones due to thermal deformation caused by a temperature change of the workpiece, even if the workpiece is machined in a uniform environmental temperature, such as a temperature-controlled room for example, wherein no thermal displacement is generated on the machine tool and the workpiece.